Shooting one of my 20” uppers and I start to have failure to feed first round. Switch magazines, same issue. Shooting PSA ammo with 77 SMK. Bolt would jump over loaded rounds or jam on feed ramps.
Gun was clean and well lubed.
I get home from range, and check bcg. Looks ok. Check recoil spring of unknown origin. It measures on the short side of spec. Rifle length buffer weighs 5.2 oz. Change recoil spring out for new Colt rifle length spring.
Back to range today. Exact same ammo, and reloads consisting of RMR 75 jhp, 24 grains Tac in new factory primed Wolf brass. Gun runs 100% Off the bench, and shooting while standing. Mags were Magpul 10 round mags. They are like new and never been a problem in multiple guns.
I can feel the difference in how smooth the gun chambered the first round and cycled.
Forgive my lack of trouble shooting experience.
Can a defective recoil spring make that much difference? Upper has had hundreds of rounds without a jam prior to this one incident.